The overall objective of this proposal is the elucidation of the basic mechanisms involved in the establishment and maintenance of a persistent infection of rhesus monkey kidney cells with simian virus 40 (SV40). Toward this end we will continue our study of the SV40 variants which arise in this system. Correlations will be sought between the capacities of the variants to promote the persistent infection and their physiological and genetic properties. Because infectious virus production is blocked in the vast majority of cells in this persistent infection, we will identify on a per cell basis the steps at which viral replication is blocked in the non-producer cells. We will also measure the fraction of nonproducing cells in which the refractory state is due to host cell rather than viral factors. We will determine the state of the latent viral genomes in non-producing clonal isolates from independent persistent infections. We will ascertain whether the frequency of virus reactivation in these clones is related to some aspect of the state of the viral genome during latent infection (e.g. episomal versus integrated or tandem duplications versus single copies at an integration site). In order to better understand why rhesus kidney cells are particularly susceptible to persistent infection by SV40, we will evaluate the role of transient host cell resistance, (which is expressed by about half of the cells in cultures of non-synchronous genetically homogeneous normal cells) in the establishment and maintenance of these infections. We will determine whether resistance is related to the cell cycle and/or to the growth state of the cells and identify the step at which viral production is blocked in the refractory normal cells.